1. Field of the Invention
The present invention relates to a fastener. More specifically, the invention relates to a self-boring and self-tapping screw.
2. Description of Related Art
A shaft of screws of the above referenced kind include a thread-free section and are also designated as partially-threaded screws. They can be screwed directly without pre-drilling into relatively soft materials, such as wood and the like. In practice, the thread-free section of the shaft may generate high friction on entering the threaded hole formed by the threaded section. This leads to a disadvantageously high driving torque.
Some existing partially-threaded screws of this kind include a shaft cutter adjacent to the thread-free section of the shaft adjacent to the threaded section. The shaft cutter includes radially projecting cutting ribs intended to enlarge the threaded hole during driving to such an extent that the thread-free section of the shaft is able to penetrate with reduced friction and a reduced driving torque. However, manufacture of such screws by a rolling process may be problematic. Deformation of the screw by bending can occur in the area of the shaft cutter during rolling. This means that a straight screw cannot be easily produced, or can only be produced with additional cost.
The object of the present invention is to make an improved self-boring screw which is capable of being driven with a low driving moment and can be produced in a simple, economical and highly accurate manner.